361 
The lactic acid fermentation of milk sugar was also investigated by 
Kichet (5), who found that when milk is kept at 40° C. it becomes 
acid and coagulates and finally attains an acidity of 1.6 per cent, 
which amount it never exceeds. He made the further interesting 
' observation that if gastric juice be added to milk the casein is coagu- 
, lated and finally dissolved, and in less than twenty-four hours the 
milk contains a larger quantity of lactic acid than otherwise would 
have been produced in a week, and after four or five days as much as 
4 per cent of lactic acid was formed. He observed that while neither a 
i’ pure solution of lactose nor gastric juice will ferment, if the two be 
j mixed fermentation takes place ; and that the casein of milk after it 
{ has been dissolved by gastric juice also ferments, yielding lactic and 
1 1 butyric acids, besides other products of fermentation. On the other 
j hand, the whey of milk obtained by coagulation with rennin never 
I attains an acidity greater than 1.6 per cent of lactic acid, even after 
I having been kept for six months. He found that the lactic acid 
: fermentation is increased by exposing a large surface of the milk to 
• the air. The activity of the ferment increases up to 44° C., remains 
i; constant between 44 and 52° C., and above 52° C. diminishes in 
I' activity as the temperature rises. Digestive juices and peptones 
' were found to aid lactic fermentation, but leucine and glycocoll 
i were found to have no effect upon the process. 
!; The general trend of more recent investigations on the subject of 
lactic acid fermentation has been to show that the change of milk 
j“ sugar into lactic acid takes place under the influence, either direct or 
indirect, of a whole series of micro-organisms, whose number has been 
considerably augmented by recent investigations in this field. Marp- 
; mann (6), for example, during the summer of 1885 investigated the 
J micro-organisms of cow’s milk in the neighborhood of Goettingen 
i and detected five seemingly new and different species of organisms 
, which more or less strongly induce the lactic acid fermentation in 
solutions of cane sugar and also in milk. 
Leaving out of consideration the levure lactique of Pasteur, the first 
) of these organisms whose morphological and biological character- 
istics seem to have been determined with sufficient accuracy is the 
Bacillus acidi lactici of Hueppe (7). It is now known that in addi- 
- tion to the Bacillus acidi lactici (Hueppe) the following organisms 
can bring about the lactic acid fermentation, viz. Bacillus aerogenes, 
i Bacillus coli. Bacillus lactis acidi (Leichmann and others), Strepto- 
1 coccus lacticus (Kruse), Streptococcus pyogenes. Pneumonococcus A 
and Pneumonococcus B, Bacillus Delbruecki (Leichmann), Bacillus 
I acidificans longissimus (Lafar), etc. 
Beyerinck (8) has also made exhaustive studies of the lactic acid 
' ferments employed in the arts. This author applies the name 
Lactobacillus Delbruecki to all species of the lactic-acid ferment 
1 ^ 
